The present invention relates to magnetic resonance imaging for diagnosis use and, in particular, to methods for eliminating ghosting artifacts in reconstructed images due to patient motion during MRI data acquisition and for increasing sharpness of the images.
There have been proposed many methods for reducing artifacts which occur in reconstructed images due to motion of the object to be imaged during MRI signal acquisition. The so-called breath holding method is a primitive method for reducing artifacts caused by patient respiration. The method imposes an excessive burden on a patient because it requires the patient to hold his or her breath during magnetic resonance signal measurement. Especially, the method cannot be effectively applied to the present day two-dimensional Fourier transform MRI techniques which take from several to several tens of minutes for manual measurement.
According to another method of a so-called gating MR method, the measuring time is substantially prolonged because the magnetic resonance signal is measured at a particular phase of patient's respiratory or cardiac cycle. However, this method requires additional equipment for monitoring breathing.
Another method called respiratory ordered phase encoding (ROPE) is described in the Journal of Computer Assisted Tomoqraphy 9 (4), pp. 835-838, 1985, wherein, a respiratory signal is used to determine the order in which rows of a data matrix for spin warp imaging are measured. Also, in this method, additional equipment is necessary for respiration monitoring
According to a method described in Radiology 1989, 173, pp. 255-263, specially encoded navigator echoes that are interleaved with an imaging sequence are measured to obtain a record of patient motion during NMR signal acquisition. The record is used for correcting displacements which would cause artifacts and unsharpness in a reconstructed image.
A further method attempting to detect motion information by using phase-encoded image data without navigator echoes and to apply appropriate compensation thereto is described in Radiology 1991, 179, pp. 139-142 and is hereafter described referred to as the "self calibration method". According to this method, encoded views forming a data matrix for reconstructing an image are sequentially detected and subjected to Fourier transformation to create a hybrid domain data set. Transformed data is used to determine object position along readout axis for each view.
The ROPE method has an advantage that the measuring time is not increased for reducing motional artifacts, because the method requires no extra scanning nor extra echoes. However, such method has a limited effect on improving the grade of image. Although ghosting artifacts that degrade the parts of the image depicting not moving structures caused by moving structures can be eliminated, blurring of the images of moving structures remains. Further, the method requires additional equipment for respiration monitoring.
For the self-calibration method, more remarkable effects on improving the image can be expected, if the determination of object position is accurate. However, because views with high-order encodings suffer from poor S/N ratio, it is difficult to accurately detect the displacements in those views. The prior art suggests a way to solve the problem by placing small diameter markers on the exterior of the patient which give clear edges on those views. The markers, however, tend to cause discomfort to the patient and the markers are also an annoyance for operators.